This invention relates to electronic apparatus including one or more memory devices for storing data and in particular to electronic apparatus including one or more so-called "Flash" memory devices.
Flash memory devices are well known in the data processing art and comprise circuits including semi-conductors designed to store electrical signals in a non-volatile manner. That is to say such memory devices continue to store signals representing data even when electrical power to the circuit is terminated. Unlike some other semi-conductor storage devices used for non-volatile storage of data, flash memory devices have the capability of being reprogrammed, i.e. the data stored in flash memory devices can be erased and new data written into the memory, without removal of the device from the electronic apparatus. Semi-conductor devices which require to be removed to enable reprogramming of the device need to be removably mounted in a device socket and this leads to increased cost of manufacture of the apparatus as well as a potential loss in reliability due to possible resistance in the interface between pins of the device and pin receiving sockets of the device socket. For reliability and reduction in cost of manufacture it is desirable for the memory device to be permanently soldered, for example by surface mounting, to electrical connections of a printed circuit board.
Flash memory devices may be programmed prior to being soldered to the printed circuit board. When a flash memory device is utilised for storing program instruction code including boot software instruction code, a microprocessor of the apparatus operating under the control of the program instruction code needs to read at least the boot code from the memory in order to perform functions including reprogramming of the memory. Accordingly in order to reprogram the memory it is necessary for the microprocessor to be able to read instruction code from the memory as well as to write new instruction code into the memory. However flash memory devices do not permit programming or reprogramming of the memory at the same time as reading data or code from the memory. Therefore it is necessary for the program instruction code, including the boot code, to be stored in another memory device from which the microprocessor can read the program instruction code so as to operate under the code stored in the other memory device to perform the operations required for reprogramming the flash memory. Therefore, in known electronic apparatus or systems incorporating flash memory where the flash memory is intended not to be removed from the apparatus for reprogramming, if it is desired to reprogram the flash memory the apparatus is provided with a separate memory device to hold `boot` software instruction code. Usually in microprocessor operated electronic apparatus, random access memory is provided for the temporary storage of data during operation of the microprocessor. Accordingly the random access memory may be used to store the program instruction code and the microprocessor is operated under control of the instruction code stored in the random access memory while the reprogramming of the flash memory is effected. Initially the microprocessor, operating under the program instruction code stored in the flash memory, reads the code from the flash memory and writes the code to the random access memory. Thus a copy of the program instruction code is stored in the random access memory. A signal then switches operation of the microprocessor from being under control of code stored in the flash memory to being under control of the code stored in the random access memory. This switching signal is a hardware signal.
It will be appreciated that the random access memory is volatile and any data or code stored therein is lost when power to the memory is terminated. While the entire program instruction code is still stored in the non-volatile flash memory, the system remains in an operational condition and when power is restored the microprocessor will be operable once again under control of the program instruction code stored in flash memory. However if the power supply fails and the supply of power to the random access is terminated, the program instruction code previously stored in the random access memory is lost and in addition the original program instruction code in the flash memory will have been wholly or partially erased and only a part of the new instruction code written to the flash memory. Accordingly as a result of loss of power and consequent shut down of the system there is no longer stored in the system a complete original program instruction code or complete new program instruction code. Without a complete program instruction code, the microprocessor is incapable of operating and the system remains inoperative until such time as the flash memory device is replaced or is reprogrammed by a different less convenient method.